Surgical sutures made from polypropylene have been successfully used by the medical profession for more than twenty years. One of the first commercially successful polypropylene suture to gain wide acceptance was described in U.S. Pat. No. 3,630,205 to Gregory J. Listner. Listner disclosed a process for manufacturing polypropylene suture that comprised the steps of drawing an extruded polypropylene suture to about 6.6 times its original extruded length in a single step and then relaxing or shrinking the monofilament to between about 91 to 76 percent of the stretched length.
Others have described sutures fabricated from polypropylene homopolymers and copolymers and from polymer blends containing polypropylene have also been described in U.S. Pat. Nos. 3,359,983, 4,520,822, 4,557,264, 4,620,542, 4,621,638 and 4,911,165.
Recently issued U.S. Pat. No. 5,217,485 describes a process for making polypropylene suture consisting of extruding, stretching (orienting), permitting the monofilament to equilibrate (age) for a period of at least 2 days prior to annealing the monofilament. The process described in the U.S. Pat. No. 5,217,485 (except for the specific storage time) is similar to the process single step draw process described in U.S. Pat. No. 3,630,205. The improvement supposedly obtained by this process was a reduced 0-5% and 0-10% strain energies, however, the data present in the patent do not support this conclusion. The strain energy exhibited by a fiber is indicative of the processing conditions used to manufacturing the fiber. Strain energy can be affected by several fiber processing steps such as the draw ratio used in orienting the fiber and any subsequent relaxation and/or annealing steps. Therefore, unless fibers with identical process histories are compared, it will be difficult to attribute decreases in strain energies to any specific factor. The U.S. Pat. No. 5,217,485 did not compare sutures with the same processing conditions, therefore, did not establish that equilibrating a suture after drawing the suture has any affect on suture properties.
It is an object of the present invention to provide an improved process for manufacturing polypropylene sutures which does not require a holding period of at least 2 days.